The present invention generally relates to the use of ultrasonic transducers to monitor flow and velocity, and more specifically, to the use of such sensors to monitor flow and velocity of blood through a cardiac valve, so as to determine the condition of the valve.
A cardiac valve that is diseased or defective can be surgically excised and replaced with an artificial valve. Once the valve has been replaced, it will be desirable to carefully monitor the condition of the valve, to ensure that it continues to function properly. While it is possible to externally examine a patient and reach some nominal diagnosis concerning the condition of a replacement valve and its proper functionality, an external examination may fail to identify incipient problems. If the valve subsequently fails catastrophically, the patient may die before surgery can correct the problem. Imaging techniques and other diagnostic evaluations of the proper functioning of an artificial valve may be impractical, patent application particularly because of expense and because they require the patient to visit facilities where such evaluations can be carried out. Accordingly, it will be apparent that means for performing such an evaluation on a continuing basis while a patient remains mobile and continues to lead a normal daily routine that does not involve frequent visits to a medical facility would be very desirable.
Often, a patient""s natural cardiac valve may be diseased, but still capable of functioning to at least a limited extent. Again, it would be helpful to monitor the condition of the natural cardiac valve on a continuous basis to enable surgery to be performed to replace the valve before the patient suffers a serious heart attack caused by a failure of the valve. Currently, the most effective technique to evaluate the condition of either an artificial cardiac valve or a natural valve is to monitor the flow of blood through the valve. However, there is no convenient way to accomplish this monitoring on a continuing basis. The velocity and volume of blood flowing through a cardiac valve are ideal parameters for determining whether the valve is functioning properly. Distal and proximal fluid pressure across a valve are a further indication of its condition.
Ideally, it would be desirable to employ means for monitoring the condition of fluid flow through a cardiac valve remotely, either continually or only periodically, upon demand. The means used for monitoring the condition of a cardiac valve should enable a physician to evaluate the parameters noted above at a remote location outside the patient""s body, without resorting to an invasive procedure. Further, the monitoring means should at least periodically be supplied power from an external source, since it is unlikely that a battery could provide the power required by sensors and circuitry required to monitor flow and other parameters indicative of a condition of a cardiac valve for an extended period of time.
Various techniques are known in the prior art for monitoring flow and velocity of a fluid inside a blood vessel, but in each case, the devices employed for this purpose are intended for relatively short-term use immediately following surgery and are not acceptable for the extended period for monitoring fluid flow, as noted above. For example, one type of volume flow measurement system described in U.S. Pat. No. 4,227,407 uses two piezoelectric ultrasonic transducers that are alternately activated to produce ultrasonic waves. The ultrasonic waves pass into a vein or artery and are modified by the flow of blood in the vessel interposed between the two transducers. When one transducer is actively transmitting an ultrasonic wave, the other transducer serves as a receiver of the wave. The two transducers are oriented at an acute angle relative to the longitudinal axis of the blood vessel, so that the ultrasonic sound wave propagating through the blood vessel has a component in the direction (or opposite to the direction) of blood flow through,the vessel. In an alternative embodiment disclosed in this patent, the transducers are located on the same side of the blood vessel, spaced apart along its longitudinal axis, and a reflective plate is disposed on the opposite side of the vessel, intermediate the positions of the two transducers. An ultrasonic wave transmitted from either transducer passes through the blood vessel, is reflected from the reflective plate, and is received by the other transducer. The difference in the transit times for the sound waves transmitted from the two transducers (in both embodiments) is indicative of the flow through the blood vessel. If transducers used only extend over a small portion of the diameter of the vessel, the difference in transit time would be indicative of the velocity of blood flowing in the blood vessel. However, since the transducers shown in this prior art reference are sufficiently large so that the diameter of the blood vessel is fully encompassed by the sound waves the transducers emit, the transit time is indicative of the flow of blood flowing through the vessel, i.e., volumetric flow. The flow is thus determined without any consideration of the internal cross-sectional area of the blood vessel. While this prior art apparatus is useful for monitoring blood flow (or velocity) through a blood vessel that is surgically exposed, the transducers are too large to be implanted within a patient""s body and are unsuitable to monitor the fluid flow status through a blood vessel associated with a cardiac valve. Also, to provide a good acoustic path between the transducers and the adjacent surface of the vessel, it may well be necessary to apply the transducers against the surface of the vessel with sufficient force to distort the wall of the vessel into the notch in the apparatus that is formed adjacent the sloping face of each transducer. Such distortion of the vessel may adversely affect the accuracy of the measurements and is undesirable over an extended period of time.
Another prior art approach for determining the velocity and/or flow of blood in a vessel employs Doppler sensing using either a pulsed or continuous wave ultrasonic signal that is emitted at a defined angle relative to the longitudinal axis of the blood vessel. If only a single transducer is used, the angle must be accurately known, and any error in the angle must be corrected. However, if a transmitting transducer is disposed on one side of the blood vessel and a receiving transducer is disposed on the opposite side of the blood vessel, angled so that the ultrasonic beam reflected from the blood flowing through the vessel is directed to the receiving transducer, an angle correction is not required.
Examples of apparatus for Doppler monitoring of blood flow are disclosed in U.S. Pat. Nos. 5,289,821 and 5,588,436. In the first of these two patents, an ultrasonic transducer wire assembly is secured to a strip of biologically inert or absorbable material, which is wrapped around and in contact with a blood vessel to form a cuff, preferably disposed downstream from an anastomosis of the vessel, such as may be performed during microvascular surgery. The wire from the transducer exits the patient""s body through a slit and is coupled to ultrasonic processing means that determine the velocity of blood flowing through the vessel by the Doppler processing of an ultrasonic wave that is transmitted by the transducer and received as a reflection from the blood in the vessel. After monitoring the velocity of blood flow for about three to seven days to determine if any thromboses has formed that would impede blood flow, the wire and transducer can be pulled from the strip and removed from the body through a small incision, leaving the strip behind. This device is not usable for an extended period of time (much beyond seven days), since the slit in the skin where the wires penetrate represents a pathway for infection. Further, the patent teaches that the invention is primarily intended for use on blood vessels close to the skin surface, such as those resulting from microvascular surgery on a patient""s hand and thus would be unusable for monitoring the fluid flow through a blood vessel adjacent to the heart and deep within a patient""s body.
In the second patent listed above, a Doppler scheme for determining blood velocity in a vessel is disclosed, wherein an elongate sheath is provided with a transducer head at its distal end. Two wires extend longitudinally through the sheath to a transducer that is mounted preferably at an angle of about 45xc2x0 relative to the longitudinal axis of the sheath. A biocompatible material such as epoxy encases both the transducer and the distal ends of the wires. This molded housing for the transducer has a concave surface that fixes the transducer relative to the blood vessel and provides a close fit to the surface of the blood vessel to provide a path for ultrasonic sound waves produced by the transducer to enter the blood vessel and for reflected waves to be detected by the transducer. A mesh band is wrapped around the transducer, and its ends are sutured together to hold the concave surface of the material in contact with the outer surface of the blood vessel. The band is made of VICRYL(trademark) mesh or other absorbable/inert material. A thread having ends that run inside and along the longitudinal axis of the sheath secure the band to the distal end of the sheath. The proximal end of the sheath is preferably left extending through the patient""s skin after the device is installed to monitor blood velocity through a vessel in contact with the concave surface of the material at the distal end of the probe. After the measurements are concluded (purportedly, after a maximum of up to 21 days), the thread is cut and pulled from its engagement with the band, so that the transducer, wires, and sheath can be withdrawn, leaving the band in placexe2x80x94possibly to be absorbed, depending on the material from which the band is fabricated.
Each of the Doppler devices discussed above is used to monitor the velocity of blood through a vessel, and to the extent that the cross-sectional area of the vessel is assumed or known, the devices enable flow to be estimated. However, neither prior art Doppler device is intended to monitor flow or velocity of blood for more than a few days. In addition, the elongate sheath used with the latter device is relatively bulky and not suitable for installation where available space around the vessel is limited. Both devices put the patient at risk of infection, because at least the wires coupled to the transducer must extend from inside the patient""s body through the skin, to an external monitoring system.
Another prior art technique for monitoring flow with a Doppler system that is more compact than the devices discussed above is based on a surface acoustic wave (SAW) transducer that couples a xe2x80x9cleaky wavexe2x80x9d into the wall of a blood vessel. The SAW transducer includes pairs of interdigital electrodes fabricated on a piezoelectric substrate that is relatively small, e.g., about 1.6 mm by 2.2 mm. This transducer is described in a paper entitled xe2x80x9cMiniature Doppler Probe Using a Unidirectional SAW Transducerxe2x80x9d by T. Matsunaka and S. Yamashita. To produce a unidirectional interdigital SAW transducer, the drive signal applied to half of the electrodes is phase shifted by 90xc2x0 relative to that applied to the other electrodes. The ultrasonic waves produced by the device propagate primarily in only one direction at an angle, xe2x96xa1, thereby enabling the direction of fluid flow in a blood vessel to be determined. The wave that would normally be transmitted in the opposite direction at an angle, xe2x88x92xe2x96xa1xe2x96xa1 is instead canceled by the interference between the interdigital electrodes driven with signals that are phase shifted relative to each other. This prior art reference states that the signal produced by a prototype SAW transducer had a maximum amplitude at a radiation angle of about 54.5xc2x0, with a beam width of about 2.5 times the actual electrode width (one mm) and suggests that the beam width might be reduced by modifying the electrode layout to achieve a xe2x80x9cfocusing effect.xe2x80x9d
Several advantages of the interdigital electrode SAW transducer design relative to the other devices available to measure flow and velocity of blood through a vessel to determine the condition of a cardiac valve are apparent. The interdigital SAW transducer is substantially smaller in size than the prior art devices and requires less energy to produce ultrasonic waves. Further, the beam width is substantially wider than the physical size of the electrodes so that the apparatus can be made relatively small compared to the size of the beam that it produces. In addition, unlike the single transducer apparatus shown in the prior art first discussed above, which produce both forward and rearwardly directed waves that are affected by the velocity of blood in either direction but cannot determine the direction of flow, the unidirectional SAW transducer is able to monitor fluid velocity and determine the direction of the fluid flow.
The prior art does not disclose an interdigital transducer that monitors transit time. Instead, each of the interdigital transducers of the prior art SAW transducer discussed above produces a leaky SAW wave and employs the Doppler effect to determine the velocity of blood in a vessel. For monitoring velocity and flow through a vessel and thereby determine the condition of a cardiac valve, it would be preferable to employ a transducer that is compact, like an interdigital SAW transducer, but one that also has the ability to measure transit time and thus flow, generally independent of any considerations of velocity profile or cross-sectional area of the vessel. This transducer should be implantable, preferably built into or secured to a supporting structure that is installed in a patient""s body, supplied with electrical power from a source outside the patient""s body, without using wires that penetrate the dermal layer, and should also permit monitoring of the flow, velocity, and pressure of a fluid without use of wires that pass through the skin. Currently, no compact prior art device is available that can remotely monitor flow and velocity parameters of a blood vessel to determine the condition of a cardiac valve for long periods (e.g., for months or even years) of time. Further, none of the prior art devices is designed to be wholly implanted, remotely monitored, and provided with power from a remote source outside the patient""s body.
In accord with the present invention, a band is defined that is adapted to be applied around a cardiac vessel to monitor a condition of a cardiac valve that controls blood flow through the cardiac vessel in a patient""s body. The band includes a biocompatible material that is sufficiently elastomeric to be wrapped around a cardiac vessel, forming a cuff. A first transducer is disposed within the wall of the cuff and produces a signal indicative of a parameter of the blood flowing through the cardiac vessel. This parameter is thus indicative of a condition of the cardiac valve. A coil is coupled to the first transducer and conveys the signal to a point external to the patient""s body for use in evaluating a condition of the cardiac valve as a function of the parameter of the blood flowing through the cardiac vessel.
The first transducer preferably includes a plurality of elements formed on a piezoelectric substrate. When excited by a radio frequency signal, the elements emit ultrasonic waves that propagate into the cardiac vessel and are affected by the blood flowing through the cardiac vessel. A receiver of the ultrasonic waves produces the signal indicative of the parameter and is coupled to the coil so that the signal produced by the receiver is conveyed outside the patient""s body.
The parameter is preferably either one of a velocity and a flow of the blood through the cardiac vessel, and the signal that is indicative of the parameter is determined as a function of the blood""s effect on the ultrasonic waves within the cardiac vessel. It is also preferably that the receiver comprise a second transducer including a plurality of elements formed on a piezoelectric substrate. The second transducer is disposed within the wall of the cuff and responds to the effect of the blood in the cardiac vessel on the ultrasonic waves to produce the signal indicative of the parameter. The first transducer and the second transducer are preferably disposed on opposite sides of the cuff, so that the ultrasonic waves pass through the cardiac vessel when traveling between the two transducers. The signal produced by the second transducer provides an indication of a transit time of the ultrasonic waves through the cardiac vessel.
The plurality of elements comprising the first transducer and the second transducer are sufficiently flexible to conform to a curved shape of an exterior surface of a cardiac vessel. In addition, the plurality of elements comprising the first transducer are divided into a first portion and a second portion, with elements comprising the first portion interdigitally dispersed among elements comprising the second portion. The elements in the first portion are adapted to couple to the radio frequency signal in one polarity, and the elements comprising the second portion are adapted to couple to the radio frequency signal with an opposite polarity, so that the ultrasonic waves produced by the elements comprising the second portion are phase shifted by about 180xc2x0 relative to the ultrasonic waves produced by the elements comprising the first portion.
Also preferably included in this embodiment is a phase shifter. The elements comprising the first transducer are then divided into four portions arranged in an ordered array in which each successive element is from a different one of the four portions, taken in order. The radio frequency signal is applied to the phase shifter, and a phase shifted signal produced by the phase shifter is applied to at least two of each successive four elements to provide about a 90xc2x0 phase difference between the ultrasonic waves emitted by successive elements. Consequently, the ultrasonic waves that are emitted by the first transducer in one direction are substantially canceled due to a destructive interference.
In another embodiment, the first transducer and the second transducer are spaced apart from each other along a side of the cuff, and a reflector is disposed on an opposite side of the cuff from the first transducer and generally opposite a point between the first transducer and the second transducer. The ultrasonic waves from the first transducer then pass through the cardiac vessel and are reflected back toward the second transducer by the reflector.
The first transducer and the second transducer may alternately function as an emitter and as a receiver of the ultrasonic waves during successive time intervals. The radio frequency signal are coupled to the plurality of elements comprising the second transducer when it functions as the emitter, while the plurality of elements comprising the first transducer are then coupled to the coil and produce the signal indicative of the parameter in response to the ultrasonic waves affected by the blood in the cardiac vessel. Also included is a multiplexer that is used for alternately coupling the first and the second transducers to the radio frequency signal and to the coil.
The frequency of the radio frequency signal is preferably controlled to determine a beam angle along which the ultrasonic waves are emitted by the first transducer, in at least one of the embodiments. The coil is adapted to couple to a source of energy that is external to the patient""s body, to provide electrical power for energizing electrical components of the band. In one embodiment, the coil is disposed within the wall of the cuff and comprises an insulated wire formed in a plurality of loops. In another embodiment, the coil is adapted to be implanted under a dermal layer in the patient""s body and to be electrical connected to the flow transducer. In either case, the coil is adapted to electromagnetically couple to an external coil that is connected to the source of energy.
The receiver preferably comprises the first flow transducer. The radio frequency signal is then applied to the plurality of element as a pulse, causing the plurality of ultrasonic waves to be emitted as a pulse. In this case, the plurality of elements comprising the first transducer receive an echo of the pulse of the ultrasonic waves that is reflected from the blood to determine the parameter based on a Doppler effect.
Another aspect of the present invention is directed to an artificial cardiac valve that monitors blood flow therethrough to determine a condition of the cardiac valve after it is mounted in a patient""s heart. The artificial cardiac valve includes a movable valve element, which in a first position, enables blood to flow in a desired direction through the artificial cardiac valve, but in a second position, blocks blood flow therethrough in an opposite direction. A generally annular support is provided for the valve element. A flow transducer is disposed within the support and is oriented to monitor flow through the artificial cardiac valve within a portion of a chamber in a patient""s heart. The flow transducer produces a signal indicative of blood flow through the artificial cardiac valve during a cardiac cycle. The blood flow can be evaluated to determine a condition of the artificial cardiac valve. The signal is adapted to couple through a radio link to an external monitoring site that is outside the patient""s body to enable the condition of the artificial cardiac valve to be monitored by medical personnel at least from time to time.
An additional transducer is preferably disposed with the support and is oriented to monitor flow through the artificial cardiac valve in a different portion of the chamber, producing an additional signal that is adapted to couple with the external monitoring device through the radio link. The additional signal is further indicative of blood flow through the artificial cardiac valve and is employed to better determine the blood flow through the valve. By using both flow transducers, the condition of the artificial cardiac valve can be more completely evaluated.
An antenna coil is disposed within the support in one embodiment. The antenna coil is used to couple energy from an external source into the flow transducer to energize it and to transmit the signal produced by the flow transducer to the external monitoring device.
In another embodiment, an implanted antenna coil is adapted to be disposed within the patient""s body, outside the patient""s heart and apart from the flow transducer, but connected thereto. The implanted antenna coil is also used to couple energy from an external source into the flow transducer to energize it and to transmit the signal produced by the flow transducer to the external monitoring device.
If multiple flow transducers are used, a multiplexer is coupled to the flow transducers and is adapted to successively couple the signals produced by the flow transducers to the radio link and the energy received from the external coil to the flow transducers to energize them.
In one preferred form of the invention, each flow transducer produces range-gated pulsed Doppler ultrasonic pulses that are usable to determine blood flow velocity through the cardiac valve. When the artificial cardiac valve is mounted in a patient""s heart, the ultrasound beams produced by the one or more flow transducers are directed into a cardiac chamber disposed upstream from the artificial cardiac valve.
Other aspects of the present invention are directed to methods that include steps generally consistent with the functions implemented by components of the apparatus discussed above.